Iron base alloys containing aluminum can have ordered and disordered body centered crystal structures. For instance, iron aluminide alloys having intermetallic alloy compositions contain iron and aluminum in various atomic proportions such as Fe.sub.3 Al, FeAl, FeAl.sub.2, FeAl.sub.3, and Fe.sub.2 Al.sub.5. Fe.sub.3 Al intermetallic iron aluminides having a body centered cubic ordered crystal structure are disclosed in U.S. Pat. Nos. 5,320,802; 5,158,744; 5,024,109; and 4,961,903. Such ordered crystal structures generally contain 25 to 40 atomic % Al and alloying additions such as Zr, B, Mo, C, Cr, V, Nb, Si and Y.
An iron aluminide alloy having a disordered body centered crystal structure is disclosed in U.S. Pat. No. 5,238,645 wherein the alloy includes, in weight %, 8-9.5 Al, .ltoreq.7 Cr, .ltoreq.4 Mo, .ltoreq.0.05 C, .ltoreq.0.5 Zr and .ltoreq.0.1 Y, preferably 4.5-5.5 Cr, 1.8-2.2 Mo, 0.02-0.032 C and 0.15-0.25 Zr. Except for three binary alloys having 8.46, 12.04 and 15.90 wt % Al, respectively, all of the specific alloy compositions disclosed in the '645 patent include a minimum of 5 wt % Cr. Further, the '645 patent states that the alloying elements improve strength, room-temperature ductility, high temperature oxidation resistance, aqueous corrosion resistance and resistance to pitting. The '645 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base alloys containing 3-18 wt % Al, 0.05-0.5 wt % Zr, 0.01-0.1 wt % B and optional Cr, Ti and Mo are disclosed in U.S. Pat. No. 3,026,197 and Canadian Patent No. 648,140. The Zr and B are stated to provide grain refinement, the preferred Al content is 10-18 wt % and the alloys are disclosed as having oxidation resistance and workability. However, like the '645 patent, the '197 and Canadian patents do not relate to electrical resistance heating elements and do not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
U.S. Pat. No. 3,676,109 discloses an iron-base alloy containing 3-10 wt % Al, 4-8 wt % Cr, about 0.5 wt % Cu, less than 0.05 wt % C, 0.5-2 wt % Ti and optional Mn and B. The '109 patent discloses that the Cu improves resistance to rust spotting, the Cr avoids embrittlement and the Ti provides precipitation hardening. The '109 patent states that the alloys are useful for chemical processing equipment. All of the specific examples disclosed in the '109 patent include 0.5 wt % Cu and at least 1 wt % Cr, with the preferred alloys having at least 9 wt % total Al and Cr, a minimum Cr or Al of at least 6 wt % and a difference between the Al and Cr contents of less than 6 wt %. However, like the '645 patent, the '109 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base aluminum containing alloys for use as electrical resistance heating elements are disclosed in U.S. Pat. Nos. 1,550,508; 1,990,650; and 2,768,915 and in Canadian Patent No. 648,141. The alloys disclosed in the '508 patent include 20 wt % Al, 10 wt % Mn; 12-15 wt % Al, 6-8 wt % Mn; or 12-16 wt % Al, 2-10 wt % Cr. All of the specific examples disclosed in the '508 patent include at least 6 wt % Cr and at least 10 wt % Al. The alloys disclosed in the '650 patent include 16-20 wt % Al, 5-10 wt % Cr, .ltoreq.0.05 wt % C, .ltoreq.0.25 wt % Si, 0.1-0.5 wt % Ti, .ltoreq.1.5 wt % Mo and 0.4-1.5 wt % Mn and the only specific example includes 17.5 wt % Al, 8.5 wt % Cr, 0.44 wt % Mn, 0.36 wt % Ti, 0.02 wt % C and 0.13 wt % Si. The alloys disclosed in the '915 patent include 10-18 wt % Al, 1-5 wt % Mo, Ti, Ta, V, Cb, Cr, Ni, B and W and the only specific example includes 16 wt % Al and 3 wt % Mo. The alloys disclosed in the Canadian patent include 6-11 wt % Al, 3-10 wt % Cr, .ltoreq.4 wt % Mn, .ltoreq.1 wt % Si, .ltoreq.0.4 wt % Ti, .ltoreq.0.5 wt % C, 0.2-0.5 wt % Zr and 0.05-0.1 wt % B and the only specific examples include at least 5 wt % Cr.
Resistance heaters of various materials are disclosed in U.S. Pat. No. 5,249,586 and in U.S. patent application Ser. Nos. 07/943,504, 08/118,665, 08/105,346 and 08/224,848.
U.S. Pat. No. 4,334,923 discloses a cold-rollable oxidation resistant iron-base alloy useful for catalytic converters containing .ltoreq.0.05% C, 0.1-2% Si, 2-8% Al, 0.02-1% Y, &lt;0.009% P, &lt;0.006% S and &lt;0.009% O.
U.S. Pat. No. 4,684,505 discloses a heat resistant iron-base alloy containing 10-22% Al, 2-12% Ti, 2-12% Mo, 0.1-1.2% Hf, .ltoreq.1.5% Si, .ltoreq.0.3% C, .ltoreq.0.2% B, .ltoreq.1.0% Ta, .ltoreq.0.5% W, .ltoreq.0.5% V, .ltoreq.0.5% Mn, .ltoreq.0.3% Co, .ltoreq.0.3% Nb, and .ltoreq.0.2% La. The '505 patent discloses a specific alloy having 16% Al, 0.5% Hf, 4% Mo, 3% Si, 4% Ti and 0.2% C.
Japanese Laid-open Patent Application No. 53-119721 discloses a wear resistant, high magnetic permeability alloy having good workability and containing 1.5-17% Al, 0.2-15% Cr and 0.01-8% total of optional additions of &lt;4% Si, &lt;8% Mo, &lt;8% W, &lt;8% Ti, &lt;8% Ge, &lt;8% Cu, &lt;8% V, &lt;8% Mn, &lt;8% Nb, &lt;8% Ta, &lt;8% Ni, &lt;8% Co, &lt;3% Sn, &lt;3% Sb, &lt;3% Be, &lt;3% Hf, &lt;3% Zr, &lt;0.5% Pb, and &lt;3% rare earth metal. Except for a 16% Al, balance Fe alloy, all of the specific examples in Japan '721 include at least 1% Cr and except for a 5% Al, 3% Cr, balance Fe alloy, the remaining examples in Japan '721 include .gtoreq.10% Al.
A 1990 publication in Advances in Powder Metallurgy, Vol. 2, by J. R. Knibloe et al., entitled "Microstructure And Mechanical Properties of P/M Fe.sub.3 Al Alloys", pp. 219-231, discloses a powder metallurgical process for preparing Fe.sub.3 Al containing 2 and 5% Cr by using an inert gas atomizer. This publication explains that Fe.sub.3 Al alloys have a DO.sub.3 structure at low temperatures and transform to a B2 structure above about 550.degree. C. To make sheet, the powders were canned in mild steel, evacuated and hot extruded at 1000.degree. C. to an area reduction ratio of 9:1. After removing from the steel can, the alloy extrusion was hot forged at 1000.degree. C. to 0.340 inch thick, rolled at 800.degree. C. to sheet approximately 0.10 inch thick and finish rolled at 650.degree. C. to 0.030 inch. According to this publication, the atomized powders were generally spherical and provided dense extrusions and room temperature ductility approaching 20% was achieved by maximizing the amount of B2 structure.
A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213, by V. K. Sikka entitled "Powder Processing of Fe.sub.3 Al-Based Iron-Aluminide Alloys," pp. 901-906, discloses a process of preparing 2 and 5% Cr containing Fe.sub.3 Al-based iron-aluminide powders fabricated into sheet. This publication states that the powders were prepared by nitrogen-gas atomization and argon-gas atomization. The nitrogen-gas atomized powders had low levels of oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders were canned in mild steel and hot extruded at 1000.degree. C. to an area reduction ratio of 9:1. The extruded nitrogen-gas atomized powder had a grain size of 30 .mu.m. The steel can was removed and the bars were forged 50% at 1000.degree. C., rolled 50% at 850.degree. C. and finish rolled 50% at 650.degree. C. to 0.76 mm sheet.
A paper by V. K. Sikka et al., entitled "Powder Production, Processing, and Properties of Fe.sub.3 Al", pp. 1-11, presented at the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh, Pa., discloses a process of preparing Fe.sub.3 Al powder by melting constituent metals under a protective atmosphere, passing the metal through a metering nozzle and disintegrating the melt by impingement of the melt stream with nitrogen atomizing gas. The powder had low oxygen (130 ppm) and nitrogen (30 ppm) and was spherical. An extruded bar was produced by filling a 76 mm mild steel can with the powder, evacuating the can, heating 11/2 hr at 1000.degree. C. and extruding the can through a 25 mm die for a 9:1 reduction. The grain size of the extruded bar was 20 .mu.m. A sheet 0.76 mm thick was produced by removing the can, forging 50% at 1000.degree. C., rolling 50% at 850.degree. C. and finish rolling 50% at 650.degree. C.
Oxide dispersion strengthened iron-base alloy powders are disclosed in U.S. Pat. Nos. 4,391,634 and 5,032,190. The '634 patent discloses Ti-free alloys containing 10-40% Cr, 1-10% Al and .ltoreq.10% oxide dispersoid. The '190 patent discloses a method of forming sheet from alloy MA 956 having 75% Fe, 20% Cr, 4.5% Al, 0.5% Ti and 0.5% Y.sub.2 O.sub.3.
A publication by A. LeFort et al., entitled "Mechanical Behavior of FeAl.sub.40 Intermetallic Alloys" presented at the Proceedings of International Symposium on Intermetallic Compounds--Structure and Mechanical Properties (JIMIS-6), pp. 579-583, held in Sendai, Japan on Jun. 17-20, 1991, discloses various properties of FeAl alloys (25 wt % Al) with additions of boron, zirconium, chromium and cerium. The alloys were prepared by vacuum casting and extruding at 1100.degree. C. or formed by compression at 1000.degree. C. and 1100.degree. C. This article explains that the excellent resistance of FeAl compounds in oxidizing and sulfidizing conditions is due to the high Al content and the stability of the B2 ordered structure.
A publication by D. Pocci et al., entitled "Production and Properties of CSM FeAl Intermetallic Alloys" presented at the Minerals, Metals and Materials Society Conference (1994 TMS Conference) on "Processing, Properties and Applications of Iron Aluminides", pp. 19-30, held in San Francisco, Calif. on Feb. 27-Mar. 3, 1994, discloses various properties of Fe.sub.40 Al intermetallic compounds processed by different techniques such as casting and extrusion, gas atomization of powder and extrusion and mechanical alloying of powder and extrusion and that mechanical alloying has been employed to reinforce the material with a fine oxide dispersion. The article states that FeAl alloys were prepared having a B2 ordered crystal structure, an Al content ranging from 23 to 25 wt % (about 40 at %) and alloying additions of Zr, Cr, Ce, C, B and Y.sub.2 O.sub.3. The article states that the materials are candidates as structural materials in corrosive environments at high temperatures and will find use in thermal engines, compressor stages of jet engines, coal gasification plants and the petrochemical industry.
A publication by J. H. Schneibel entitled "Selected Properties of Iron Aluminides", pp. 329-341, presented at the 1994 TMS Conference discloses properties of iron aluminides. This article reports properties such as melting temperatures, electrical resistivity, thermal conductivity, thermal expansion and mechanical properties of various FeAl compositions.
A publication by J. Baker entitled "Flow and Fracture of FeAl", pp. 101-115, presented at the 1994 TMS Conference discloses an overview of the flow and fracture of the B2 compound FeAl. This article states that prior heat treatments strongly affect the mechanical properties of FeAl and that higher cooling rates after elevated temperature annealing provide higher room temperature yield strength and hardness but lower ductility due to excess vacancies. With respect to such vacancies, the articles indicates that the presence of solute atoms tends to mitigate the retained vacancy effect and long term annealing can be used to remove excess vacancies.
A publication by D. J. Alexander entitled "Impact Behavior of FeAl Alloy FA-350", pp. 193-202, presented at the 1994 TMS Conference discloses impact and tensile properties of iron aluminide alloy FA-350. The FA-350 alloy includes, in atomic %, 35.8% Al, 0.2% Mo, 0.05% Zr and 0.13% C.
A publication by C. H. Kong entitled "The Effect of Ternary Additions on the Vacancy Hardening and Defect Structure of FeAl", pp. 231-239, presented at the 1994 TMS Conference discloses the effect of ternary alloying additions on FeAl alloys. This article states that the B2 structured compound FeAl exhibits low room temperature ductility and unacceptably low high temperature strength above 500.degree. C. The article states that room temperature brittleness is caused by retention of a high concentration of vacancies following high temperature heat treatments. The article discusses the effects of various ternary alloying additions such as Cu, Ni, Co, Mn, Cr, V and Ti as well as high temperature annealing and subsequent low temperature vacancy-relieving heat treatment.